In the semiconductor industry, integrated circuits (ICs) are typically tested before release to market. This testing process often involves using probe machines to check numerous die (ranging from 200 to 2,000, maybe more) on a silicon wafer. The dies, if not rejected, are then cut off and mounted on a semiconductor device.
Many conventional probe machines use a probe card that makes contact with the silicon wafer circuitry as it checks for electrical current resistance. The probe card typically contains 16 to 500 or more probe needles set at an angle relative to the plane defined by the surface of the probe card. The wafer is mounted on a chuck and is contacted by the probe card. This contact and other environmental factors introduce debris, such as metal, polymers and organo-metallic materials, into the probe card. As debris accumulates, the performance of the probe card can be adversely affected. Accordingly, to maintain proper functioning of the probe card, the probe card should be polished free of debris periodically using a probe polishing assembly.
In some conventional probe polishing assemblies, a chuck descends and a stage holding the chuck and the probe polish assembly moves to place a burnishing pad under the probe card. A pneumatic cylinder then lifts the burnishing pad to a position suitable for polishing the probe needles. The burnishing pad then burnishes the probe needles.
For purposes of this discussion, movements of the various components involved in the polishing process can be considered as occurring in three dimensions designated using the familiar XYZ coordinate system. The polishing process involves slightly raising the probe polish assembly in the Z direction. After each thrust in the Z direction, the probe polish assembly is subjected to slight movements in the X and Y directions. The stage on which the polish assembly and chuck are mounted performs the movements in the Z direction at a programmed height and frequency. The stage also executes the movements in the X and Y directions. After the probe card is polished, the burnishing assembly lowers and the probe card returns to a position suitable for probing.
In polishing the probe card, the top surface of the circular burnishing pad should be as coplanar as possible to the plane of the probe needles. Deviations from this norm can prevent some needles from receiving adequate burnishing. If needles are inadequately burnished, the remaining debris potentially introduces contact resistance, which in turn can lead to false measurements. These false measurements can cause the tester to reject the integrated circuit, decreasing the wafer yield. Noncoplanarity between the top surface of the circular burnishing pad and the plane of the probe needles can also cause some needles to be burnished too much. Excessive burnishing wears the needles rapidly and can damage the probe itself.
In some conventional approaches, the burnishing pad is manually planarized using a gauge that measures differences in planarity between the chuck and burnishing pad at different points. Adjustments are made in an attempt to bring these differences to within a tolerance of 0.5 mil from a point on the perimeter of the burnishing pad's top surface to a diametrically opposed point. It can be appreciated that these adjustments are precise and time-consuming.
Adjusting the burnishing pad typically involves fixing a stem, which loosely fits inside an aluminum housing and supports the burnishing pad, into place. The burnishing pad is fixed into position by tightening three Allen-type set screws onto the stem. The set screws' center lines radiate from the center of the housing at 120 degree intervals. Accordingly, the orientation of the burnishing pad can be varied by tightening the set screws to different degrees.
With many burnishing pad arrangements, however, the stem is cylindrical, as illustrated at reference numeral 20 in FIGS. 1 and 3. As illustrated in FIG. 2, a conventional set screw 50 has a concave tip 56 that defines a circle at its distal end. This tip only makes contact with the vertical surface of the stem at no more than two points, namely, the 90 and 270 degree points on the circle. This lack of contact increases the likelihood that the set screws will slip or chip the stem. Moreover, when the cylindrical stem is not vertically aligned, contact with the set screws is limited to one point, further increasing the likelihood that the set screws will slip or chip the stem. When the set screws slip, the top surface of the burnishing pad ceases to be adequately planarized. The operator might not notice this problem for some time, causing wafer yields to decrease dramatically as more ICs are erroneously rejected. In addition, once the problem is found, the burnishing pad must be replanarized, taking the probe device out of commission for hours or even days.